The interaction between purified platelet fibrinogen receptor and fibrinogen in the presence and absence of calcium and/or solubilizing detergents has been studied by analytical centrifugation. A mathematical model for the time course of separation of macromolecules on self-forming cesium chloride density gradients has been developed. A statistical-thermodynamic model has been developed for the effect of excluded volume due to fibrous elements of the cytomatrix upon macromolecular equilibria in the fluid phase of cytoplasm.